This invention relates in general to a system for modulation of beams of charged particles, and in particular to a gate used for such purpose and a process for making the gate.
For many experiments, it is necessary to deflect the trajectory of a beam of charged particles. One of the most convenient methods for accomplishing this task is to use an interleaved comb of wires, which is called a Bradbury-Nielson gate (BNG). A BNG consists of two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. When no potential is applied to the wires relative to the energy of the charged particles corresponding to an xe2x80x9conxe2x80x9d state of the beam, the trajectory of the charged particle beam is undeflected by the gate, as illustrated in FIG. 1A. To deflect the beam from its original trajectory, bias potentials of equal magnitude and opposite polarity are applied to the two individual wire sets. Deflection produces two separate beams, each making an angle xcex1 with respect to the path of the undeflected beam as shown in FIG. 1B. In this manner, it is possible to modulate or gate ion beams in a controlled fashion.
Bradbury-Nielson gates were developed as electron filters decades ago As these gates had a much smaller effective field size than the commonly used deflection plates, Bradbury-Nielson gates have been used for modulating ion beams in time-of-flight mass spectrometry (TOF-MS). Since that time, many groups have reported similar use. A common application is mass-to-charge (m/z) selection in time-of-flight mass spectrometry (TOF-MS). Ions are allowed to drift before reaching the gate where short xe2x80x9con pulsesxe2x80x9d allow only ions of a selected mass-to-charge to pass. Tandem configurations, where the rising and falling edges of the ion packets are created by two different BNGs, have been described as a way to improve mass resolution for m/z selection. Use of BNGs is also common in ion mobility mass spectrometry, where the gates regulate the injection of ion packets into the drift tube.
An extremely demanding application for these gates is Hadamard transform time-of-flight mass spectrometry (HT-TOFMS). In HT-TOFMS, the ion beam is modulated with a pseudo-random sequence of xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d pulses by applying the corresponding modulation to a Bradbury-Nielson gate. After the pseudo-random sequence is applied, the ion packets created by the on/off modulation interpenetrate one another as they drift through the flight tube. The detected signal is a convolution of the mass spectra corresponding to these packets. Using knowledge of the applied pseudo-random sequence, this signal is deconvoluted to yield a single mass spectrum.
In order to improve mass resolution and modulation pulse profiles, much effort has been made to produce Bradbury-Nielson gates with minimal spacing between wires. A detailed description of the use of this device in time-of-flight mass spectrometry appeared in 1995 by Vlasak et al. See xe2x80x9cAn interleaved comb ion deflection gate for m/z selection in time-of-flight mass spectrometry,xe2x80x9d by P. R. Vlasak et al., Rev. Sci Instrum., 1996, 67, 68-72. In this work, a wire spacing of 1 mm was achieved by weaving a wire through holes on two separate frames and applying tension with a bracing screw between the two frames. A significant reduction of the wire spacing to 0.5 mm was reported in 1998 by Stoermer et al. who used the grooves on two nylon threads to control the wire spacing. This group used two sequential grids to minimize pulse widths. Still, they concluded that further reduction in wire spacing would improve m/z selectivity in TOF experiments.
The next advance in the reduction of the wire spacings was reported by Brock, Rodriguez, and Zare, who were able to construct Bradbury-Nielson gates for their HTM-TOF mass spectrometer with a wire spacing of 0.16 mm, working by hand under a microscope to set the wires in a frame made from a piece of printed circuit board (PCB) and aligned by means of two threaded rods fixed to opposite ends of the PCB. This procedure was extremely laborious, requiring several days to complete the assembly of a single gate. Furthermore, the frames were expensive and the quality of the fabricated grids was inconsistent. It is therefore desirable to provide improved BNG and other gates used for modulating a beam of charged particles, and an improved method for making these gates.
This invention is based on the observation that the above difficulties are alleviated by providing a body having a surface, a hole through the surface and grooves on the surface to serve as alignment vehicles for the wires during the winding process. The body has also at least a first and a second electrical contact, preferably on or near the body. An electrically conducting wire is wound under tension onto the grooves. As a result, a first set of portions of the wire in grooves that are not adjacent to one another is in contact with the first electrical contact, and a second set of portions of the wire is in at least some of the remaining grooves are in contact with the second electrical contact. The first set is attached to the first electrical contact, and the second set is attached to the second electrical contact so that the portions of the two sets pass over one side of the hole through the surface. The grooves and the surface of the body are such that the two sets of wires are substantially co-planar at the hole.
An improved gate for electrically modulating a beam of charged particles comprises a body having a surface and grooves on the surface, a hole in the body through the surface and at least a first and a second electrical contact on or near the body. A first set of electrically conducting wires located in grooves that are not adjacent to one another but are in electrical contact with the first electrical contact. The gate also comprises a second set of electrically conducting wires located in at least some of the remaining grooves on the surface and in electrical contact with the second electrical contact. The two sets of portions of wires pass over one side of the hole at the surface. The grooves and surface are such that the two sets of wires are substantially co-planar at the hole.
Grooves that are in sections and wire positioning guides other than grooves may also be used instead. By employing such guides on the same body, it would be much easier to make or fabricate such guides so that they provide co-planar alignment for the wire. Preferably, the sectional grooves and guides are on the same surface of the body.
The invention also provides a method comprising winding electrically conducting wire under tension about a body. The body has a surface and a plurality of wire-positioning features along the surface. The wires are wound so that the positioning features maintain a first set of portions of the wire interspersed with a second set of portions of the wire across the surface. The two sets are electrically isolated from each other, and the first set of portions of the wire are attached to a first electrical contact and the second set of portions of the wire to a second electrical contact. The positioning features will often comprise groves in the surface of the body, may be defined by discrete protrusions extending from the surface, or the like.
When the device formed by any one of the methods described above is used, a beam of charged particles transiting a hole through the surface of the body is modulated using electrical potentials applied to the first and second electrical contacts while the portions of the first and second sets of the wire span the hole and are substantially co-planar at the hole.
In another aspect, the invention provides a gating apparatus for electrically modulating a beam of charged particles, the apparatus comprises a body having a surface and a hole in the body through the surface. A first set of electrically conducting wires are in electrical contact with a first electrical contact. A second set of electrically conducting wires are interspersed with the first set with a spacing between adjacent wires of the two sets being about 1 mm or less, said second set being in electrical contact with a second electrical contact, the two sets being electrically isolated from each other, wherein the two sets of portions of wires span the hole along the surface such that the two sets of wires are substantially co-planar at the hole. Optionally, the wires spanning the hole are in tension.
Each of the above-described gate and gating apparatus is preferably also provided with a driver unit for applying electrical potentials to the first and second electrical contacts in order to modulate the beam of charged particles transiting a hole through the surface of the body.